Independent of their constitution, which can cover a number of possible variations from aliphatic to fully aromatic, polyesters and copolyesters are generally produced in a two-stage process. In the first stage, the esters to undergo polycondensation, or a polyester precondensate comprising a mixture of oligoesters and having an average relative molecular weight of normally 100-2,000 depending on the molar ratio of the starting compounds, are produced by transesterification of dicarboxylic acid esters or esterification of dicarboxylic acids with an excess of dialcohols. If a branching modification is desired, limited amounts of higher-functional starting components such as glycerin, pentaerythritol, or trimellitic acid can also be employed. Equivalent process methods for the first stage are the conversion of dicarboxylic acid chlorides with diols, the attachment of ethylene oxide to dicarboxylic acids, the esterification of an anhydride with a dialcohol, the conversion of anhydrides with epoxides, and the conversion of dicarboxylic acids or dicarboxylic acid esters with the diacetate of a diol. The second reaction stage is the actual polycondensation, in which the desired high molecular weight of the polyesters and copolyesters must be attained through splitting off alcohol and/or water. In addition to applying a vacuum, introducing an inert gas, and increasing the reaction temperature, polycondensation is accelerated in particular by specific polycondensation catalysts.
For the production of film and fiber-forming polyesters, a legion of polycondensation catalysts have been proposed to accelerate the polycondensation reaction. Since the great majority of the compounds cited in numerous patents have an insufficient catalytic activity or other disadvantages, compounds containing Sb have found almost exclusive use as polycondensation catalysts in the art. Unfortunately, this catalyst has recently encountered criticism on environmental grounds, so that a replacement generally appears to be desirable.
Attempts are constantly being made to supply catalysts to replace Sb.sub.2 O.sub.3. In particular, alkoxy titanates, especially tetrabutyl titanate, have been proposed, whereby these compounds are used either for transesterification only (JA-PS 74 11 474), transesterification and polycondensation (JA-OS 77 86 496), or polycondensation only (JA-OS 80 23 136), since they are catalytically active in both stages. Since the use of titanium compounds causes discoloration of the polycondensed polyesters, JA-OS 78 106 792 requires pretreatment of titanium compounds with various organic substances, e.g., amines, or they must be combined with other polycondensation catalysts, in particular with Sb.sub.2 O.sub.3 (JA-OS 78 109 597).
DE P 947 517 teaches that metallic oxides such as zinc oxide, boron trioxide, lead oxide, and titanium dioxide can be used as polycondensation catalysts for producing polyethylene terephthalate. The polycondensation time with these metallic oxides, however, is inordinately long, from 7-14 hours in the examples given in that publication. For this reason, BE P 619 210 uses Sb.sub.2 O.sub.3 (see example 1) as a polycondensation catalyst to supplement TiO.sub.2 for producing the polyesters described therein, which dramatically increases the speed of the polycondensation process. Given these circumstances, it of course became practical to work only with Sb.sub.2 O.sub.3 or titanium tetrabutylate as a polycondensation catalyst (see the additional examples of BE P 619 210).
DE-A1 44 00 300 and DE-A1 44 43 648 disclose TiO.sub.2 /SiO.sub.2 and TiO.sub.2 ZrO.sub.2 coprecipitates as polycondensation catalysts.